If successful, the latest variants of the worm turn off security controls in the Windows firewall and open a backdoor onto compromised machines, allowing hackers to remotely control machines, which thereafter become zombie clients in botnet networks.

Previous versions of the worm caused two earlier Windows vulnerabilities to spread, as explained in an advisory by CA here.

Microsoft last week released a "critical" patch for the Windows server flaw exploited by Cuebot-L and Cuebot-M. Security experts were quick to see its potential for exploitation, now realised with the Cuebot-L and Cuebot-M worms.

The Department of Homeland Security took the unusual step of warning of the seriousness of the flaw shortly after Redmond's release of the corresponding software fix. ®